Current-limiting fuse with dual element release and having extensions of the current-limiting elements cut by arc blasts in arc chutes and by a mechanical cutter



S. l. LINDELL Feb. 14, 1967 CURRENT-LIMITING FUSE WITH DUAL ELEMENT RELEASE AND HAVING EXTENSIONS OF THE CURRENT-LIMITING ELEMENTS CUT BY ARC BLAST AND BY A MECHANICAL CUTTER Filed Jan. 27, 1966 S IN ARC GHUTES 2 Sheets-Sheet 1 KJ TY if @0% @www f T0 LOWE/ F055 ERM/NAL Feb. 14, 1967 s. l. LINDELL 3,304,390

CURRENT-LIMITING FUSE WITH DUAL ELEMENT RELEASE AND HAVING ExTENsIoNs CE THE CURRENT-LIMTTING ELEMENTS CUT BY AEC BLAsTs IN ARC CHUTEs AND BY A MECHANICAL CUTTER Filed Jan. 27, 1966 2 Sheets-Sheet 2 3,304,390 CURRENT-LIIVHTING FUSE WITH DUAL ELEMENT RELEASE AND HAVING EXTENSIUNS F THE CURRENT-LIMITING ELEMENTS CUI` BY ARC BLASTS IN ARC CHUTES AND BY A MECHANI- CAL CUTTER Sigurd L. Lindell, Northbrook, Ill., assigner to S it C Electric Company, Chicago, Ill., a corporation of Delaware Filed Jan. 27, 1966, Ser. No. 523,442 18 Claims. (Cl. 24W-120) This invention relates, generally, to fuses and it has particular relation to current-limiting fuses for use on alternating current circuits operating at voltages at and above 2,400 volts in which the normal current flow may range from 3 to 200 amperes. It constitutes an improvement over the current-limiting fuses described in my applications as follows: Serial No. 522,676, tiled January 24, 1966; Serial No. 523,228, filed January 26, 1966; Serial No. 523,312, filed January 27, 1966; and lassigned to the assignee of this application.

In the application rst above referred to a currentlimiting fuse is described having current calibrated fusible element means located in air Iwithin one of the fuse tube terminals and connected in series circuit relation with a plurality of parallel connected -current-limiting fusible elements located in a filling of granular relatively inert material, such as sand, and having a common terminal at the junction `with the calibrated fusible element means. Expressed in magnitude of current, the time-current curve for the current calibrated fusible element means lies below the time-current curve for the combined parallel connected current-limiting fusible elements while the time-current curve for an individual one of them lies below the time-current curve for the current calibrated fusible element means. In order to cause the currentlimiting fusible elements to blow one by one on flow of relatively low fault current sufficient to blow the current calibrated fusible element means, spark gaps -are pro-vided one for each current-limiting fusible element. These `spark gaps are connected in parallel with the gap that is formed on blowing of the current calibrated fusible element means. One electrode of each spark gap is connected to the junction between a major portion of the respective current-limiting fusible element and a minor portion or extension thereof. One of the spark gaps break-s down at random on-blowing of the current calibrated fusible element means and the fault current divides between the major portion of the associated current-limiting fusible element `and the minor portion -or extension thereof which now is connected in series with the other parallel connected current-limiting fusible elements. As a result, Ia larger portion of the fault current iiows through the extension or minor portion and it blows or melts.' Then the total fault current flows through the major portion of the current-limiting fusible element and causes it to blow or melt at several points of reduced cross section along its length. Tlhereafter another of the spark gaps breaks down at random and this sequence is repeated until all of the current-limiting fusible elements have blown or melted and the circuit is finally completely opened.

An improvement over the current-limiting fuse of the first application above referred to is set forth in the application second above referred to. According to it the several minor portions or extensions of the current-limiting fusible elements are positioned in arc blast chutes in which the respective spark gaps also are located. These minor portions or extensions are positively cut or severed one 4by one mainly as the result of the heat and vapor heat generated by arcs formed at the `spark gaps added to the heat generated by current flowing therethrough. Then United States Patent O it is unnecessary to depend entirely upon the flow of a part of the fault current for opening the circuits through the minor portions or extensions of the current-limiting fusible elements.

In the applicaiton third above referred t-o there is described a further improvement over the current-limiting fuse of the rst application. A spring biased mechanical impact member is employed for simultaneously severing the several minor portions or extensions of the current-limiting fusible elements. It is released on blowing 0f the current calibrated fusible element means. This action effectively opens the circuits through the minor portions or extensions of the current-limiting fusible elements on relatively low fault currents without requiring that a part of the fault current be used for melting or blowing them.

Among the objects of this invention are: In currentlimiting fuses of the types referred to above to provide for increasing the time delay in the blowing of the current calibrated fusible element means for relatively low fault current below the current-limiting range to facilitate coordination with other circuit interrupting `devices in the circuit; to employ a relatively slow current calibrated fusible element to respond to and initiate the clearing of relatively low fault currents in series with la relatively fast current-limiting section to respond to and interrupt high 'fault currents by current-limting `action before the calibrated element blows; as expressed in current magnitude to position the low current long time end of the time-current characteristic of the current calibrated fusible element below the time-current characteristic of the current-limiting section but having the short time end of the Vlatter substantially below the short time end of the former; to provide separate current responsive means to release an indicator before the current-limiting section has cleared a high fault current; to provide an indication that the current-limiting fuse has operated whether as a result of 'a low current fault or of a high current fault and `whether or not the current calibrated fusible element means has blown; to movably mount the intermediate terminal to which one end of the current calibrated fusible element means is connected and to restrain it by an auxiliary fusible element that is connected in shunt with the minor portion or extension of one of the currentlimiting fusible elements 'and in series with the major portion thereof; to employ the arc chutes of the Asecond above identified application in conjunction with the dual element current calibrated fusible element means described herein; and to employ the mechanical impact member of the third above identied 'application in conjunction with the dual element current calibrated fusible element means disclosed herein.

In the drawings: FIG. 1 is a vertical section-al view, substantially at `full scale, of a current-limiting fuse embodying one species of this invention. FIGS. 2, 3 4and 4 are horizontal sectional views taken generally along the lines 2 2, 3 3 -and 4 4 of FIG. 1. FIG. 5 is a top plan view of the ceramic arc chute member. FIG. 6 is a vertical sectional view taken `generally along the line 6 6 of FIG. 5. FIG. 7 illustrates diagrammatieally the circuit connections embodied in the current-limiting fuse shown in FIG. 1. FIG. 8 shows graphically the Arelation between the melting times of the various current carrying elements of the current-limiting fuse for different current tiows therethrough. FIG. 9 shows graphic-ally the current fiow when the available short circuit magnitude of alternating current is so high that the current-limiting section melts on rise of current and the current ow is interrupted in a fraction of the first .current loop by the current-limiting fuse embodying this invention. FIG. 10 is a vertical sectional view, substantially -at full scale, Iof another current-limiting fuse embodying this invention. FIG. 11 is a horizontal sectional view taken generally along the line 11-1'1 of FIG. 10. FIG. 12 is a vertical sectional view taken generally along the line 12-'12 of FIG. 11. FIG. 13 is a horizontal sectional view take-n lgenerally .alon-g the line 13-13 of FIG. 10. FIG. 14 is a top plan view of the ceramic `guide member shown in FIG. 15. FIG. 15 is a view, in side elevation, of the ceramic guide member shown in FIG. 14. FIG. 16 is a view, in side elevation, of the combined metallic indicator tube and impact member. FIG. 17 is a bottom plan view of the combined metallic indicator tube and impact member shown in FIG. 16.

In FIG. 1 the `reference character 10 designates, generally, a current-limiting fuse which includes an elongated insulating housing 11 of ceramic material such as porcelain. It may be formed of other insulating material. The elongated -insulating housing 11 has a fuse upper end terminal indicated, generally, at 12 and a `fuse lower end terminal indicated, generally at 13. Each of the end terminals 12 and 13 includes a metallic ferrule 14 that is intended to be positioned in suit-able fuse clips. Each metallic `ferrule 14 has an indented portion -15 that tits into a :groove 16 in the housin-g `11. Flan-ges 1'7 extend radially inwardly from the ends of the metallic ferrules 14 in overlying Irelation to connector plates 18 that are welded thereto and overlie the respective ends of the housing 11. Gaskets 119 are interposed, as s-hown, in order to prevent egress of inert granular material 20, such as sand, with which the housing 11 is iilled.

The lower end terminal 13 includes a connector member 23 which has la radial ange 24 that overlies the flange 17 on the adjacent metallic ferrule 14. The lower end of the fuse housing |11 is closed by a cover plate 25 after the relatively inert granular material has been inserted. Screws 26, extending through the peripheral portion of the cover plate 25 and the flanges 24 and l17 into the connector plates 18, .serve to hold the several parts tightly in position. A threaded stud 2'7 extends through the central portion of the connector member 23 into the lower end of. a Ceramic core that is indicated, generally, at 28 :and extends upwardly through the housing `11. The core 2S is provided with four vertical ribs 29 which are grooved as indicated at 30.

The current-limiting portion of the current-limiting fuse 10 is formed, for example, zby `four current-limiting fusible elements or ribbons 31 which are disposed in the grooves 30 and wound spirally around the ribs 29 of the ceramic core 28. Larger or `smaller numbers of currentlimiting fusible elements or ribbons 31 can be employed. The current-limiting fusible elements or ribbons 31 are formed of silver, silver alloy or nickel. They are provided with perforations 32 that are uniformly spaced .from end to end to provide localized restricted cross sections to initiate fusion and arcing on ow of fault current. Terminal strips 33 have their ends 34 crimped to the lower ends of the current-limiting fusible elements or ribbons 31. The terminal strips 33 extend crosswise lof the low-er end of the ceramic core 28 with the threaded stud 27 extending therethrough. A nut 35 serves to securely :cl-amp the central portion of the connector member 23 to the yjuxtaposed porti-ons of the terminal strips 33. 'Ihis -construction provides a relatively low resistance connection from the lower ends of the current-limiting fusible elements -or ribbons 31 to the lower end terminal 13 o-f the current-limiting fuse 10.

The upper ends of the current-limiting fusible elements or ribbons 31 extend through and are securely connected to terminal fittings 38 and thereby constitute a junction to lwhich reference will be made hereinafter. The terminal fittings 38 are secured in a radial iiang-e 39 of a ceramic arc chute member indicated, generally, at 4t) and shown in more detail in FIGS. 5 `and 6. The arc chute member 40 has a central opening 41 that is square in cross seci tion for receiving a correspondingly shaped stem 42 that forms a part of an intermedi-ate terminal which is indicated, generally, at 43 and is movably mounted near the upper end of the insulating housing 11. The stern 42 extends downwardly into the upper end of the central aperture 41 in the .a-rc chute member 40.

The intermediate termin-al 43 is held against upward movement by lan auxiliary fusible element or strain wire 44 which is secured `at its upper end to the stem 42 and extends downwardly through the centr-al opening 41. Its lower end is anchored to `an auxiliary terminal 45 which is secured by a transverse anchor pin 46 to the reduced upper end portion 47 of the ceramic core 28. The intermediate terminal `43 includes an enlarged ybody portion 48 which is slidably mounted -at the lower end of a guide tube 49 that is formed preferably of ceramic insulating material. The guide tube 49 has an integral radially extending flange 50 at its lower end.

The intermediate terminal 43 also includes a lower fuse element terminal 51 to which the lower end of a current calibrated fusible element means, shown generally at 52, is connected. The current calibrated fusible element means 52 includes a coiled silver fusible element 53 and a str-ain wire S4. They are secured at their upper ends to an upper fuse element terminal 55 which extends upwardly through the top 56 of a metall-ic indicator tube 57 which is slidably mounted on the guide tube 49. The metallic indicator tube 57 is biased `from the non-indicating position shown in FIG. 1 to an indicating position by a coil compression spring 58. It is interposed between the top 56 of the metallic indicator tube 57 and the upper end of the guide tube 49. As long as t-he current calibrated lfusible element means 52. remains intact or unblown, the

spring 58 is restrained `and the metallic indicator tube 57 is held in the non-indicating position.

The upper fuse element terminal 55 also extends through a dished washer 59 and a nut 60 threaded on the upper end serves to hold the assembly in position. A garter spring 61 of berylium copper is interposed between the underside of the dished washer 59 and a conducting washer 62 to complete the current path to the upper end terminal 12. Screws 63 extend through the w-asher 62 `and into the connector plates 18 to complete the assembly. A stop ring 64 is secured to the metallic indicator tube 57 intermediate its ends for limiting its upward movement to the indicating position. With a view to increasing voltage that appears between the fuse element terminals 51 and 55 when the current calibrated fusible element means 52 blows, lit is surrounded by a ber sleeve 65 which is secured at its upper end to the upper fuse element terminal 55.

Provision is made for extending the connection from the upper end terminal 12 into the upper end of the insulating housing 11. For this pur-pose a ange 67 is positioned under the washer 62 and is clamped thereto by the screws 63. The flange 67 extends radially outwardly from the upper end of a metallic tube 68` which projects inwardly of the insulating housing 11 and is surrounded by the relatively inert granular materia'l 20, such as sand. The lower end of the metallic tube 68 is stepped at 69 for receiving the periphery of the flange 39 which forms an integral part of the ceramic arc chute member 40;

Those portions of the current-limiting fusible elements or ribbons 31 which are located between the lower end terminal 13 and the terminal fittings 38, FIG. 7, are referred to herein and in the claims as m-ajor portions 70` while the portions 71 that project upwardly from the terminal fittings 38 are referred to herein and in the claims as minor portions or extensions 71. The latter at 72 extend through the body port-ion 48 of the movable intermediate terminal 43 and are secured thereto by crimping. Each terminal tting 33 is connected to the currentlimiting fusible element or ribbon 31 at the junction be tween the perforated major portion 70 and perforated minor portions or extensions 71.

It will be observed that cavities or arc blast chutes 73 are lformed between the anges 3.9 and 50 of the ceramic arc chute member 40l and of the guide tube 49. The lower ends of the arc blast chutes 73 are closed off by the flange 39 and their upper ends open into the space within the guide tube 49. Since the terminal fittings 39 are mounted in the insulating flange 39, they are insulated from the juxtaposed lower end portion of the metallic tube 68. The spaces therebetween form spark `gaps 74, there being a spark 'gap 74 for each of the arc blast chutes 73 through each of which one of the perforated minor portions or extensions 71 of the current-limiting fusible elements or ribbons 31 extends. Preferably, each of the arc blast chutes 73 is provided with gas evolving material 75 for the purpose of increasing the blast action of the arc that is formed at the respective spark gap 74. The material 75 can be an inorganic material such as calcium carbonate, magnesium borate or the like.

On low current operation on the vformation of an arc at any of the spark gaps 74, there is a ow of arc plasma and a blast action therefrom as indicated by arrows 76. The arc Iblast impinges on the perforated minor portion or extension 71 in the respective anc blast chute 73 and it is promptly severed at the perforation 32 therein to interrupt the current ow therethrough.

Under normal operating conditions with the currentlimiting fuse '10 in the unblown condition, the current path is from the upper end terminal 12, through the garter spring 61 to the upper fuse element terminal 55. The current path continues through the current calibrated fusible element means 55 to the movable intermediate terminal 43 and thence through the current-limitinrg fusible elements or ribbons 31 in parallel to the lower end terminal 13. As seen in FIG. 7, a part of this current path extends through the auxiliary fusible element or strain wire 44 that is connected between the movable intermediate terminal 43 and stationary auxiliary terminal 45 in parallel with one of the minor portions or extensions 71.

FIG. 8 shows the time-current characteristic for the several fusible elements. Here curve 78 represents the time-current relationship for the current calibrated fusible element means 52. It is calibrated so as to melt or blow as indicated by solid line curve 78 and by its response at the long time end of this curve to determine the ampere rating of the current-limiting fuse which rating is conventionally approximately One-'half of the iive minute or ten minute melting current. The position of the curve 78 Iat its short time end indicates that the time-current characteristic of the current calibrated fusible element means 52 is relatively slow at intermediate and high currents with respect to the time-current characteristic of the combined current-limiting fusible elements or ribbons 31 to provide time delay at low and intermediate fault currents for coordination with other circuit interrupters, such as secondary breakers used in conjunction with distribution transformers. Broken line curve 79 indicates the time-current relationship for the blowing of all four of the parallel connected current-limiting fusible elements or ribbons 31. In lgeneral terms, the time-current characteristics shown by curve 78 is relatively slow while the time-current characteristic shown by curve 79 is relatively yfast and they intersect. This indicates that the arrangement is such that the current calibrated fusible element means 52 at the high current short time end of curve 78 would not blow or melt on flow of fault current before blowing or melting of the current-limiting fusible elements or ribbons 31 has occurred and the current has been interrupted by their current-limiting action. The succeeding curves, shown by broken lines 80, 81 and 82, show the time-current characteristics for the blowing of 3, 2 and 1, respectively, of the current-limiting fusible elements or ribbons 31. The overall time-current characteristic of the current-limiting fuse 10 is shown by the upper end 78' of the curve 78 and the lower end 79' of the curve 79 which is illustrated by a heavy broken line. To simplify the presentation the curves 78-82 here shown illustrate typical average R.M.S. current values. It is understood that allowance for plus or minus deviations are to be made with sufficient margin for coordination with other series connected current interrupting means and with sections within the current-limiting fuse 10.

In some cases the flow of fault current may be so high that the circuit will be interrupted by the blowing of the current-limiting fusible elements or ribbons 31 without blowing or melting of the current calibrated fusible element means 52 and Without the formation of arcs at the spark gaps 74. This is shown by dotted line curve 83 in FIG. 8 for all four of the current-limiting fusible elements 31. Curve 83 in FIG. 8 shows the clearing time for one of the current-limiting fusible elements 31. Under such operating conditions, if the intermediate terminal 43 were stationary, the metallic indicator tube 57 would not be released to show that the circuit had been interrupted by the current-limiting fuse 10. It is for this reason that the auxiliary fusible element or strain wire 44, which preferably is formed of relatively high resistance, high strength Nichrome wire and has a minimum current carrying capacity compatible with the tensile strength required to restrain the indicator tube 57, is provided to melt or blow when the currentlimiting fusible elements or ribbons 31 melt or blow and clear the circuit.

Curve 84 in FIG. 9 indicates the instantaneous current in a half loop of alternating current that is available to ow in the event of a high current fault provided some means is not introduced to limit such current flow to a value well below the maximum here indicated. Assuming that the current flow is as indicated at I3 in FIG. 8 at time t5, the current-limiting fusible elements or ribbons 31 blow substantially simultaneously. Because they are embedded in the filling of relatively inert granular material 20, such as sand, the resistance of the circuit provided thereby rises rapidly so that the fault current I3, instead of following the curve 84, FIG. 9, rises only to a relatively low peak value and is rapidly decreased. At time t6 it is reduced to zero.

In the event that the current calibrated fusible element means 52 does not blow under the fault conditions just described, after blowing of the minor portion or extension 71 in parallel with the auxiliary fusible element or strain wire 44, the current flow shifts to it and it blows at time t7 in FIGS. 8 and 9. This action, which occurs immediately following the melting of the current-limiting fusible elements 31 at the perforations 32 and before the flow of fault current has been interrupted thereby, releases the intermediate terminal 43 for movement together with the unblown or intact current calibrated fusible element means 52 under the inuence of the spring 58 and the metallic indicator tube 57 is moved endwise of the insulating housing 11 to indicating position.

The above described coordinated melting or rupture of the auxiliary strain element 44 is provided at all current levels above I2 up to the highest current-limiting level, i.e., for all time intervals shorter than t3. During the melting period at these current levels the current is divided substantially equally between the parallel current-limiting fusible elements 31. This is illustrated in FIGS. 8 and 9. For current level I3 in the currentlimiting region, the equal division may be substantially maintained during the arcing period until the current is interrupted as shown in FIGS. 8 and 9. At current levels above I2 but below the current-limiting levels, when the melting time may be appreciable, the current is also divided approximately equally between the parallel current-limiting fusible elements 31. A difference in melting time may cause current to shift from one of these elements to another during the arcing period which may extend through several current zeros, while the initial short arch at the perforations 32 melt part or all of the unfused portions of the respective element until final interruption takes place. There is suficient margi in time and` current while arcing occurs in the currentlimiting fusible element 31 with which the auxiliary fusible element or strain wire 44 is paired up to insure its melting or rupture to release the metallic indicator tube 57.

The time-current characteristics of the fusible elements of the current-limiting fuses disclosed in the above application are coordinated to effect release of the indicator at all fault levels. This does not permit the additional coordination that can be obtained employing fusible clements having the time-current characteristics as disclosed herein with which the auxiliary fusible element or strain wire 44 is employed to release the indicator tube 57 under certain conditions as herein disclosed.

As pointed out in the application first above referred to particular diiculty of interrupting the current is encountered when the ow of fault current is substantially lower. For example, it is conventional to rate a fuse to blow at a minimum value at approximately twice its normal current carrying rating. Such a fault current is indicated at I1 in FIG. 8. In that case it will be observed that the flow of fault current is not suicient to -blow all four of the current-limiting fusible elements or ribbons 31 but it would be suliicient to blow one of them as indicated at t2 along curve 82. By providing the spark gaps 74 in the manner described in said first application, it is possible successively to blow the major portion 70 of the current-limiting fusible elements or ribbons 31 so that, under the assumed conditions where the current flow is as indicated at I1, they will be blown one by one.

As disclosed in the application second above referred to, the current calibrated fusible element means 52 is blown under conditions of relatively low fault current, such as the current I1 at time t1 in FIG. 8, and the voltage across the resulting gap between the fuse eiement terminals 51 and 55 is applied across the spark gaps 74 which, as shown in FIG. 7, are connected in parallel. One of them will break down at random so that an are is formed therebetween, for example as indicated at 87. The action will be made more clear from a consideration of FIG. l which represents the conditions that exist when the arc 87 is drawn between the terminal fitting 38 shown to the left and adjacent the lower end of the metallic tube 68. The arc 87 is formed at the lower end of the arc blast chute 73. There is a tendency for the arc 87 to progress along the arc blast chute 83 and to impinge upon the adjacent part of the minor portion or extension 71 therein which is promptly severed. As a result the entire flow of fault current then takes place through the arc 87 and the associated current-limiting fusible element or ribbon 31 which blows or melts out at several perforations 34 therealong. This operation is repeated at random among the three remaining spark gaps 74, and under the low fault current conditions represented by the current Il, the succeeding current-limiting fusible elements or ribbons 31 are blown one by one.

At the instant that the first spark gap 74 breaks down and the arc 87 is formed, one-fourth of the fault current flows through the perforated major portion 70 of the respective current-limiting fusible element or ribbon 31 and three-fourths of the fault current flows through the respective perforated minor portion or extension 71 and through the other three current-limiting fusible elements or ribbons 31. While this amount of fault current owing through the perforated minor portion or extension 71 may be sufficient to cause it to blow or melt, this action is made much more certain by the provision of the arc blast chute 73 and the arrangement for causing the arc 87 positively to cut the adjacent perforated minor portion or extension 71, without depending upon this portion or extension to melt as a result of ow of a portion of the fault current therethrough. This action is enhanced through the provision of the arc extinguishing material '75. Since the `arc blast chutes 73 and the portions or extensions 71 of the current-limiting fusible elements or ribbons 31 extending therethrough are segregated from the iilling of inert granular material 20, such as sand, the cutting of the minor portions or extensions 71 sequentially in the manner described is unimpeded by the presence of the filling material 20.

In FIG. l0 the reference character 100 designates, generally, another embodiment of a current-limiting fuse that embodies the features of the current-limiting fuse disclosed in the application last above referred to. The current-limiting fuse includes an elongated insulating housing 101 of ceramic material such as percelain. It may be formed of other insulating material. The elongated insulating housing 101 has a fuse upper end terminal indicated, generally, at 102 and a fuse lower end terminal indicated, generally, at 103. The upper end terminal 102 includes a metallic ferrule 104- that is secured by cement or other means to the upper end of the housing 101. The ferrule 104 has connector plates 105 weldedl thereto for receiving screws 106 that extend through a radial flange 107 that forms an integral part of a metallic terminal cap 108. The end terminal 102 also includes a metallic tube 109 that extends downwardly into the tubular insulating housing 101 and is provided with a radially inwardly extending ange 110'. It will be understood that the metallic ferrule 104, metallic terminal cap 108 and the metallic tube 109 are solidly interconnected and that the metallic terminal cap 108 is arranged to be positioned in a fuse clip. Gaskets, shown collectively at 111, serve to prevent the egress of relatively inert granular material 112, such as sand, which fills the tubular insulating housing 101.

The lower end terminal 103 includes a metallic ferrule that is suitably secured by cement to the lower end of the insulating housing 101. It has connector plates 116 welded thereto for receiving screws 117 that extend through a flange 118 which extends outwardly from a metallic terminal cap 119. The metallic terminal cap 119 has an opening 120' through which the granular material 112 can be introduced. A closure cap 121 is provided for the opening 120. The lower end terminal 103 also includes a connector plate 122. Gaskets, shown collectively at 123, are employed to prevent the egress of the granular material 112. It will be understood that the ferrule 115, metallic terminal cap 119, andthe connector plate 122 are solidly interconnected and that metallic terminal cap 119 is arranged to be received by a fuse clip.

Extending centrally of the tubular insulating housing 101 is a ceramic core that is indicated, generally, at 126. The ceramic core 126 is provided with a central opening 127 and has four vertical ribs 128 that are provided with grooves 129 for receiving in spiral fashion preferably four or more current-limiting fusible elements or ribbons 130 which may be formed of silver, silver alloy or of a relatively high melting point higher resistivity metal such as nickel for current-limiting fuses of low ampere rating. Perforations 131 are uniformly spaced along each of the current-limiting fusible elements 130 to provide localized restricted crosssections to initiate fu' sion and arcing on ow of fault current. The lower ends of the current-limiting fusible elements or ribbons 130 are Aconnected by terminal clips 132 secured to the connector plate 122 by screws 133. Thus the lower ends of the current-limiting fusible elements or ribbons 130 are solidly connected to theV fuse lower end terminal 103.

As shown at the upper end of FIG. 10 and in FIG. 12 `the upper ends of the current-limiting fusible elements or ribbons 130 extend through metallic terminals 137 that .are generally tubular in form and are suitably crimped to provide good contact engagement with the Current-limiting fusible elements or ribbons 130. The terminals 137 extend through sleeves 138, FIGS. 14-15, that are formed integrally with a flange 139 that forms a part of a guide member, shown generally at 140, which has a central upstanding guide tube 141. A gasket 142 underlies the flange 139 and serves to prevent egress of the granular material 112 from the insulating housingl 101.

As shown in FIGS. 10, 1l and 12 the current-limiting fusible elements or ribbons 130 extend horizontally as indicated at 143 from the upper ends of the terminals 137 and into intimate contact with a body portion 144 of an intermediate terminal that is indicated, generally at 145, and is slidably mounted in the guide tube 141. The body portion 144 is suitably crimped to the ends of the current-limiting fusible elements or ribbons 130.

As pointed out, in accordance with this invention, the intermediate terminal 145 is slidably mounted in the guide tube 141. It is restrained from upward movement therein by an auxiliary fusible element or strain wire 146 which is secured at its upper end to an extension 147 which depends from the body portion 144 of the intermediate terminal 145. The lower end of the auxiliary fusible element or strain wire 146 is secured in an auxiliary terminal 148 which has a radial flange 149 that bears against a shoulder 150 at the top of a counter bore in the flange 139 of the guide member 140. A conductor 151 connects the auxiliary terminal 148 to one of the current-limitingfusible elements or ribbons 130 for a purpose to be described. Slots 152 are provided in the guide tube 141 to receive the horizontal portions 143 of the current-limiting fusible elements or ribbons 130.

The intermeditae terminal 145 also includes a lower fuse element terminal 153 to which the lower end of current calibrated fusible element means, shown generally at 154, is connected. It includes a coiled silver fusible element 155 and a strain wire 156 with their upper ends being secured to an upper fuse element terminal 157 that projects through an apertured ange 158 of a metallic indicator tube 159. A nut 160, threaded on the upper end of the upper fuse element terminal 157 serves to hold the assembly in place as long as the current calibrated fusible element means 154 and the auxiliary fusible element or strain wire 146 remain unblown.

The upper end of the metallic indicator tube 159 extends into a flanged opening 161 in the metallic terminal cap 108. Contact engagement is maintaned with the metallic indicator tube 159 when it moves from the nonindicating to indicating position by contact fingers 162 which are secured by screws 163 to the radially inwardly extending flange 110 from the metallic tube 109. The screws 163 project into the upper ends of the vertical ribs 128 of the ceramic core 126. The metallic indicator tube 159 has an enlarged lower end portion 164 which forms a shoulder 1615 that is arranged to limit the upward movement of the metallic indicator tube 159| as it is urged upwardly by a coil compression spring 166 on blowing of the current calibrated fusible element means 154. The coil compression spring 166 is positioned between the underside of the apertured flange 158 and a stepped metallic ring 167 which overlies the upper end of the guide tube 141. A ber fuse tube 168 is secured to the upper fuse element terminal 157 and extends downwardly over the current calibrated fusible element means 154. The fiber fuse tube 168 is employed for evolving an arc extinguishing medium `on blowing of the current calibrated fusible element means 154 and for increasing the arc voltage between the fuse element terminals 1153 and It is desirable that provision be -made for blowing the current-limiting fusible elements or ribbons 130 one by one, particularly on flow of relatively low fault current. In order to accomplish this terminal conducting strips 170 are connected to a junction 1.71 between a minor portion or extension 172 of the respective current-limiting fusible element or ribbon 130, which includes the horizontal portion 143, and a major portion 173 of the respective current-limiting fusible element or ribbon 130. The arrangement is generally similar to that illustrated diagrammatically in FIG. 7 with reference to the currentlimiting fuse 10. The terminal conducting strips 170 are held in place by screws 174 that are threaded into the respective vertical rib 128 of the ceramic core 126. Each of the terminal conducting strips 170 is provided at its upper end with an electrode 175 that extends into an opening 176 in the ange 139 of the guide member 140. Above each opening 176 there is a vent opening 177 in the radially inwardly extending flange of the metallic tube 109. This arrangement provides a spark gap 178 between the electrode 175 and the adjacent portion of the flange 110 which constitutes the other electrode.

On blowing of the current calibrated fusible element means 154 as the result of ow of fault current therethrough or on blowing of the auxiliary fusible element or Strain wire 146, the spring 166 no longer is restrained. It moves the metallic indicator tube 159 upwardly until the shoulder 165 engages the underside of the top of the metallic terminal cap 108. This indicates that the currentlimiting fuse 100 has blown and should be replaced. The blowing of the current calibrated fusible element means 154 is accompanied by the formation of a gap between the fuse element terminals 153 and 157 across which a voltage appears that is applied to the spark gaps 178. -One of them at .random breaks down and an arc is formed at this spark gap. This arc corresponds to the arc 87, FIG. 7, that is formed under similar conditions in the operation of the current-limiting fuse 10. One fourth of the fault current then flows through the major portion 174 of the respective current-limiting fusible element or ribbon while three fourths of it flows through the respective minor portion vor extension 172 thereof to the intermediate terminal and thence through the remaining three current-limiting elements or ribbons 130. While this amount of fault current flowing through the perforated minor portion or extension 172 may be sufficient to cause it to blow or melt, this action is made more certain by use of a mechanical impact member, shown generally at 181 and in detail in FIGS. 16 and 17. The mechanical impact member 181 preferably is formed of insulating material, such as a ceramic material, and it has a cylindrical body portion 182 that telescopes into the enlarged lower end portion 164 of the metallic indicator tube 159. The cylindrical body portion 182 has an annular groove 183 for receiving an indented portion 184 of the enlarged lower end portion 173. At its lower end the cylindrical body portion 182 has slots 185 to permit application of the mechanical impact member 181 with respect to the horizontal portions 143 of the current-limiting fusible elements or ribbons 130 and positioning integrally formed teeth 186 the upper cutting or impacting edges 187 of which are arranged to engage the undersides of the horizontal portions 143 under the influence of the coil compression spring 166 to sever them mechanically immediately upon blowing of the current calibrated fusible element means 154. Preferably the spring 166 is capable of exerting a force of the order of 10 pounds. Since the horizontal portions 143 are arranged to be relatively taut, they can be severed readily by the mechanical impact member 181 when it moves upwardly under the influence of the coil compression spring 166.

Thus it is unnecessary to rely upon the transfer of fault current to the minor portions or extensions 173 of the current-limiting fusible element or ribbons 130 for blowing them by fault current flowing therethrough and transferring the flow of fault current back to the major portion 173 in each case. Instead, on blowing of the current calibrated fusible element means 154, the min-or portions or extensions 172 are promptly severed mechanically by the mechanical impact member 181. The voltage then appearing at the gap formed by the blowing of the current calibrated fusible element means 154 is applied across the spark gaps 178 in parallel. One of them at random breaks down and the fault current then flows through the respective major portion 173 until it blows at several points therealong as determined by the locations of the perforations 131. The arc voltage then is transfererd to the remaining spark gaps 178 and they break down at random followed by the successive blowing `of the remaining current-limiting fusible elements or ribbons 130.

Under noi-mal operating conditions with the currentlimiting fuse 100 in the unblown condition, the current path is from the upper end terminal 102 through the contact fingers 162 to the metallic indicator tube 159 and thence through upper fuse element terminal 157, the cur-rent calibrated fusible element means 154, intermediate terminal 145 and current-limiting fusible elements or ribbons 130 to the lower end terminal 103.

The current path also includes the auxiliary fusible element or strain wire 146 which interconnects the intermediate terminal 145 and the auxiliary terminal 148, with the latter being connected through conductor 151 to the junction 171 between the minor portion or extension 172 and the major portion 173 of one of the currentlimiting fusible elements or ribbons 130. Thus the auxiliary fusible element or strain wire 146 is connected in parallel circuit relation with the minor portion or extension 172 of one of the current-limiting fusible elements or ribbons 130.

The reason for the provision of the auxiliary fusible element or strain wire 146 is that, as pointed out above, in some cases the flow lof fault current may Ibe so high that the -circuit will be interrupted by the blowing of the current-limiting fusible elements 130 without blowing or melting of the current calibrated fusible element means 154. Under such operating conditions, as indicated hereinbefore, if the intermediate terminal 145 were held stationary, the metallic indicator tube L159 would not be released to show that the circuit had been interrupted by the current-limiting fuse 100.

In the event that the current calibrated fusible element means 154 does not blow under fault conditions that result in the blowing of the current-limiting fusible elements or ribbons 130, after blowing of the minor portion or 172 in parallel with the auxiliary fusible element or strain wire 146, the current iiow shifts to it and it blows at time t7 shown in FIGS. 8 and 9. The intermediate terminal 145 then is no longer restrained and it moves upwardly together with the unblown current calibrated fusible element means 154 under the influence of the coil compression spring 166. Also, the metallic indicator tube 159 is moved upwardly of the insulating housing 101 to indicating position.

Since the curves shown in FIGS. 8 and 9 of the drawings and described hereinbefore with reference to the operation -of the current-limiting fuse 10 are also applicable to the operation of the current-limiting fuse 100, a description thereof will not be repeated with respect to the functioning of the current-limiting fuse 100.

It will be understood that the provision of the currentlimiting fuse disclosed in the application first above referred to can be constructed to have time-current characteristics as shown by FIGS. 8 and 9. In such case the auxiliary f-usib'le element or strain wire 44 or 146 can be employed in the manner described herein to insure release of the indicator under all fault current conditions.

What is claimed as new is: i

1. A current-.limiting fuse including:

an elongated insulating housing with end terminals,

an intermediate terminal in said housing insulated from said end terminals,

fusibile element means interconnecting one end terminal and said intermediate terminal,

a plurality of current-limiting fusible elements ernbedded in an inert filling in said housing and connected in parallel circuit relation between said intermediate terminal and the other end terminal,

a minor portion of each current-limiting fusible element adjacent said intermediate terminal constituting 4an extension of a major portion thereof,

means providing a spark gap individual to each currentlimiting fusible element, one terminal of each spark gap being connected to the junction between the respective extension and the major portion of its current-limiting fusible element, the other terminal of each spark gap including conductor means connected to said one end terminal thereby placing said spark gaps in shunt circuit relation to the gap formed on blowing of said fusible element means -on iiow of fault current therethrough,

an auxiliary terminal connected t-o the junction of one of said current-limiting elements,

means interconnecting said intermediate terminal and said auxiliary terminal, and

means responsive to blowing of at least one vof said current-limiting fusible elements for interrupting the connection between said intermediate terminal and said auxiliary terminal.

2. The current-limiting fuse according to claim 1 wherein:

the intermediate terminal is movably mounted,

spring means act through the fusible element means to bias said intermediate terminal, and

the means interconnecting said intermediate terminal and the auxiliary terminal prevents movement of said intermediate terminal by said spring means.

3. The current-limiting fuse according to claim 2 wherein the means interconnecting the intermediate terminal and the auxiliary terminal comprises auxiliary fusible element means.

4. The current-limiting fuse according to claim 3 wherein an indicator is movable endwise of the elongated housing, is biased to indicating position 4by the spring, and is restrained in non-indicating position as long as both the fusible element means and the auxiliary fusible element means remain intact.

5. The current-limiting fuse according to claim 1 wherein means responsive to blowing of the fusible element means sever the extensions of the major portions of the current-limiting fusible elements.

6. The current-limiting fuse according t-o claim I wherein means individual to each extension provide an arc blast chute in which the extension is located.

7. The current-limiting fuse according to claim 4 wherein each arc blast chute has at least a portion of its inner surface formed by a material capable -of evolving a gaseous medium when subjected to the heat of the arc formed at the respective spark gap for increasing the blast action in the chute.

8. The current-limiting fuse according to claim 1 wherein means operated in response to blowing of the fusible element means mechanically severs the extensions.

`9. The current-limiting fuse accord-ing to claim 8 wherein:

the severing means is a mechanical impact member movable endwise of the elongated housing, and

spring means restrained by fusible element means and the auxiliary fusible element means bias said mechanical impact member to sever said extensions.

10. The current-limiting fuse according to claim 9 wherein at least the impact portion of the impact member is formed of insulating material.

11. The current-limiting fuse according to clalm 9 wherein the means interconnecting the intermediate terminal and auxiliary terminal comprises auxiliary fusible element means.

12. The current-limiting fuse according to claim 11 wherein an indicator is movable endwise of the elongated housing conjointly with the impact member from non-indicating position to indicating position on Iblowing of either the fusible element means or the auxiliary fusible element means.

13. The current-limiting fuse according to claim 1 wherein each currentlimiting fusible element is provided with substantially uniformly spaced reduced cross sections to provide locations at which it yblows on flow of fault current.

14. In a current-limiting fuse, in combination:

current calibrated fusible element means,

a movable terminal connected to said current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected to said movable terminal and thereby in series circuit relation with said current calibrated fusible element means,

a spring,

a movable indicator biased by said spring to indicating position and restrained by said current calibrated fusible element means,

a stationary auxiliary terminal connected to a junction between a minor portion of one of said current-limiting fusible elements and the major portion thereof,

an auxiliary fusible element interconnecting said movable and stationary auxiliary terminals and restraining said indicator from movement by said spring in tandem wtih said current calibrated fusible element means, and

means responsive to blowing of said current calibrated fusible element means for causing said current-limiting fusible elements to blow one by one,

said indicator being released for movement to indicating position on blowing of said current calibrated fusible element means, and

said auxiliary fusible element being blown on flow of relatively high fault current suliicient to blow said current-limiting fusible elements without blowing said current calibrated fusible element means whereby said indicator is released for movement to indieating position.

15. In a current-limiting fuse, in combination:

current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected in series circuit relation with said current calibrated fusible element means,

the current values of the long time end of the timecurlrent characteristic curve for :said current calibrated fusible element means lying below the current values of the long time end of the time-current characteristic curve for said plurality of parallel connected currentlimiting fusible elements,

the current values of the short time end of said timecurrent characteristic curve for said current calibrated fusible element means lying above the current values of the short time end of said time-current characteristic curve for said plurality of parallel connected current-limiting fusible elements, and

means responsive to blowing of said current calibrated fusible element means at current Values along said long time end of its time current characteristic curve including a spark gap individual to and connected in series circuit relation with each current-limiting fusible element and connected in parallel circuit relation with said current calibrated fusible element for caus- 1 4 ing said current-limiting fusible elements to blow one by one.

16. In a current-limiting fuse, in combination:

current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected in series circuit relation with said current calibrated fusible element means,

the current values of the long time and of the tirne-current characteristic curve for said current calibrated fusible element means lying below the current values of the long time end of the time-current characteristic curve for said plurality of parallel connected currentlimiting fusible elements,

the current values of the short time end of said timelcurrent characteristic curve for said current calibrated fusible element means lying above the current values of the short time end of said time-current characteristic curve for said plurality of parallel connected current-limiting fusible elements,

means responsive to blowing of said current calibrated fusible element means at current values along said long time end of its time-current characteristic curve for causing said current-limiting fusible elements to blow one by one,

indicator means biased from non-indicating to indicating position and restrained from movement as long as said current calibrated fusible element means remains intact,

auxiliary means cooperating with said current calibrated fusible element means to restain said indicator means, and

means responsive to blowing of said current-limiting fusible elements without blowing of said current calibrated fusible element means for releasing said indicator means.

17. The current-limiting fuse according to claim 1S wherein z wherein z means connect each spark gap in series circuit relation with a major portion of the respective current-limiting fusible element and to the current calibrated fusible element means through the respective minor portion of said current-limiting fusible element, and

means operated in response to blowing of said current calibrated fusible element means for mechanically severing said minor portions.

References Cited by the Examiner UNITED STATES PATENTS 2,400,408 5/ 1946 Haefelfnger 200-118 2,435,472 2/ 1948 Schuck 200-121 2,502,992 4/ 1950 Rawlins et al 200-120 3,218,517 ll/1965 Sankey 317-66 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

1. A CURRENT-LIMITING FUSE INCLUDING: AN ELONGATED INSULATING HOUSING WITH END TERMINALS, AN INTERMEDIATE TERMINAL IN SAID HOUSING INSULATED FROM SAID END TERMINALS, FUSIBLE ELEMENT MEANS INTERCONNECTING ONE END TERMINAL AND SAID INTERMEDIATE TERMINAL, A PLURALITY OF CURRENT-LIMITING FUSIBLE ELEMENTS EMBEDDED IN AN INERT FILLING IN SAID HOUSING AND CONNECTED IN PARALLEL CIRCUIT RELATION BETWEEN SAID INTERMEDIATE TERMINAL AND THE OTHER END TERMINAL, A MINOR PORTION OF EACH CURRENT-LIMITING FUSIBLE ELEMENT ADJACENT SAID INTERMEDIATE TERMINAL CONSTITUTING AN EXTENSION OF A MAJOR PORTION THEREOF, MEANS PROVIDING A SPARK GAP INDIVIDUAL TO EACH CURRENTLIMITING FUSIBLE ELEMENT, ONE TERMINAL OF EACH SPARK GAP BEING CONNECTED TO THE JUNCTION BETWEEN THE RESPECTIVE EXTENSION AND THE MAJOR PORTION OF ITS CURRENT-LIMITING FUSIBLE ELEMENT, THE OTHER TERMINAL OF EACH SPARK GAP INCLUDING CONDUCTOR MEANS CONNECTED TO SAID ONE END TERMINAL THEREBY PLACING SAID SPARK GAPS IN SHUNT CIRCUIT RELATION TO THE GAP FORMED ON BLOWING OF SAID FUSIBLE ELEMENT MEANS ON FLOW OF FAULT CURRENT THERETHROUGH, AN AUXILIARY TERMINAL CONNECTED TO THE JUNCTION OF ONE OF SAID CURRENT-LIMITING ELEMENTS, MEANS INTERCONNECTING SAID INTERMEDIATE TERMINAL AND SAID AUXILIARY TERMINAL, AND MEANS RESPONSIVE TO BLOWING OF AT LEAST ONE OF SAID CURRENT-LIMITING FUSIBLE ELEMENTS FOR INTERRUPTING THE CONNECTION BETWEEN SAID INTERMEDIATE TERMINAL AND SAID AUXILIARY TERMINAL. 